Ultra low expansion silica-titania glass such as ULE® glass (Corning Incorporated) is the material of choice for mirror substrates for use in Extreme Ultraviolet Lithography (EUVL) tools due to its extremely low coefficient of thermal expansion (CTE). The ULE substrates are specified within a very narrow range for the value of the average zero CTE crossover temperature, Tzc. While Tzc can be controlled by glass composition, which is defined at the time of glass forming, it can also be affected by the thermal history of the glass. Control of the composition during the glass forming stage is not always sufficiently accurate such that a particular sample or boule of glass will satisfy the glass requirements for a certain part if the specification for Tzc is too narrow. For example, in combustion processes where silica-containing and titanium-containing feedstocks are fed (mixed or not mixed) into burners, combusted into silica and titania oxides, deposited in a vessel and formed into a glass, the plugging of the burners (partial or complete in one or more burners) or variations pump rates (perhaps due to a voltage variation or pump problem) can cause some variations in the formed glass composition.
In the near future it is envisioned that the Tzc requirements will be tightened and that the size of the glass substrates required for EUVL substrates will increase. The difficulty in meeting these future requirements will not only be affected by any problems that may be encountered in the glass manufacturing process, but will be further compounded by the increase in size of the parts. As a result, it is also highly desirable to have a method by which the Tzc of a silica-titania glass EUVL mirror (or other) substrate can be tuned to within the desired specification range. In addition, since is desirable that the different mirrors in a set of mirrors can be specified at different values of Tzc, it is further desirable that the method be suitable to make the different Tzc required for each mirror formed from a single boule of glass, or different mirrors made from a single large glass sample, thus reducing the number of separate boules or large samples required to fill an order. That is, multiple mirror parts can be shaped from a single boule or large sample of glass and then the Tzc of each individual part adjusted as needed.